A pressure-sensitive pointing device installed in a keyboard of a notebook PC, and so forth is made up such that when a user presses down an operation console of the device in a desired direction with a fingertip, a load applied in that direction is detected by a strain sensor built inside the device, and a detection signal of the strain sensor is processed, whereupon a pointer such as a cursor, or the like, shown on a display of the notebook PC, is caused to shift. At this point in time, a shifting direction of the pointer is determined correspondingly to a direction of the load applied to the tip of the device, and a shifting speed is determined correspondingly to magnitude of the load.
As a conventional signal processing system for processing output signals of a pressure-sensitive pointing device (hereinafter referred to also as a pointing device), there is available an input unit as disclosed in Patent Document 1. FIG. 9 is a block diagram showing the input unit.
An output signal of a pressure-sensitive pointing device 131 is inputted to such a signal processing system 121. The pressure-sensitive pointing device 131 comprises a strain sensor 131a for detecting a load in a plus direction along an x-axis (hereinafter referred to as a +X direction), a strain sensor 131b for detecting a load in a minus direction along the x-axis (hereinafter referred to as a−X direction), a strain sensor 131c for detecting a load in a plus direction along a y-axis (hereinafter referred to as a +Y direction), and a strain sensor 131d for detecting a load in a minus direction along the y-axis (hereinafter referred to as a −Y direction), the respective loads resulting from an operation of an operation console, not shown. In this case, the x-axis refers to an axis in a side-to-side direction or in the lateral direction relative to the operation console of the pointing device 131, as seen from a user, while the y-axis refers to an axis in a front-to-back direction or the longitudinal direction relative to the operation console of the pointing device 131. Further, the x-axis corresponds to a side to side direction, or the lateral direction, on a display of a notebook PC, and so forth, in which the pointing device 51 is installed, and the y-axis corresponds to a front-to-back direction or the longitudinal direction, on the display.
The strain sensors 131a, 131b, 131c, 131d each are made up of a strain gauge such as a piezoelectric resistance element, and when the operation console, not shown, is operated in the +X direction, the −X direction, the +Y direction, and the −Y direction, respectively, the strain sensors 131a, 131b, 131c, 131d each are pressed downward corresponding to respective directions of operations, and respective resistance values thereof undergo a change due to the respective loads applied thereto. The strain sensors 131a, 131b are connected in series, and the strain sensors 131c, 131d are connected in series. Such series-connected circuits as described are connected in parallel, forming a parallel-connected circuit, and a power supply voltage Vdd is fed to the parallel-connected circuit. The four strain sensors, under no load, are equal in resistance value, however, when the operation console is pressed down in the +X direction, the −X direction, the +Y direction, and the −Y direction, respectively, the resistance value of any of the strain sensors 131a, 131b, 131c, 131d, positioned in the in the direction in which the operation console is pressed down, undergoes a change, whereupon a strain along the x-axis direction is detected as a voltage change via a node 131e between the strain sensors 131a, 131b, while a strain along the y-axis direction is detected as a voltage change via a node 131f between the strain sensors 131c, 131d. If the operation console is pressed down in a slanting direction (a direction within a plane containing the x-axis, and the y-axis, but nonparallel to the x-axis and the y-axis) at this point in time, there are detected a stain corresponding to an x-axis direction component of a vector in a direction in which the operation console is pressed down, and a stain corresponding to a y-axis direction component of the vector. Upon removal of the load, the respective resistance values of the strain sensors revert to the respective resistance values thereof, under no load, and potentials at the nodes 131e, 131f, respectively, revert to respective values before the voltage change. The nodes 131e and 131f are connected, respectively, to terminals 121a and 121c of the signal processing system 121.
Low-pass filters 132, 133 comprise capacitors 132a, 133a, and resistors 132b, 133b, respectively, and with the low-pass filters 132, 133, an upper cut-off frequency is set to on the order of 150 Hz so as to remove low frequency noise components out of respective output signals of operational amplifiers 123, 124, to be described later. Further, an output side of the low-pass filter 132 is connected to terminals 121a, 121b of the signal processing system 121, respectively, and an output side of the low-pass filter 133 is connected to terminals 121c, 121d of the signal processing system 121, respectively.
The signal processing system 121 comprises a digital processing circuit 122 having a CPU 122a, a ROM 122b, a RAM 122c, for executing control of the signal processing system 121 in whole, and so forth, the operational amplifier 123 having an inverting input side connected to the terminal 121a, and a noninverting input side connected to an output side of a digital-to-analog converter (hereinafter referred to as a DAC) 126 to be described later, an output side of the operational amplifier 123 being connected to the terminal 121b, the operational amplifier 124 having an inverting input side connected to the terminal 121c, and a noninverting input side connected to an output side of a DAC 127 to be described later, an output side of the operational amplifier 124 being connected to the terminal 121d, an analog switch SW 19 connected to the output side of the operational amplifier 123, an analog switch SW 20 connected to the output side of the operational amplifier 124, an analog-to-digital converter (hereinafter referred to as an ADC) 125 having an input side connected to a common output side of the analog switches SW 19, SW 20, and an output side connected to an input side of the digital processing circuit 122, the DAC 126 having an input side connected to an output side of the digital processing circuit 122, and the output side connected to the noninverting input side of the operational amplifier 123, and the DAC 127 having an input side connected to the output side of the digital processing circuit 122, and the output side connected to the noninverting input side of the operational amplifier 124. The low-pass filters 132, 133 serve as feedback circuits of the operational amplifiers 123, 124, respectively.
There is described hereinafter an operation of the signal processing system 121 having such a configuration described as above.
The voltage corresponding to the strain along the x-axis direction, outputted from the node 131e of the pointing device 131, is inputted from the terminal 121a to the inverting input side of the operational amplifier 123. Similarly, the voltage corresponding to the strain along the y-axis direction, outputted from the node 131f of the pointing device 131, is inputted from the terminal 121c to the inverting input side of the operational amplifier 124. Reference data outputted from the digital processing circuit 122 is converted into an analog reference voltage by the DAC 126 to be subsequently inputted to the noninverting input side of the operational amplifier 123. The reference data outputted from the digital processing circuit 122 is converted into the analog reference voltage by the DAC 127 to be subsequently inputted to the noninverting input side of the operational amplifier 124. Now, assuming that the strain sensors 131a, 131b, 131c, 131d, under no load, each have a resistance value Rs, and the resistors 132b, 133b of the low-pass filters 132, 133, respectively, each have a resistance value Rf, the operational amplifiers 123, 124 each have a gain of −{Rf/(Rs/2)}, so that a change (on the order of +10 mV) in the voltage corresponding to the strains along the x-axis, and the y-axis, respectively, can be amplified to a voltage change (on the order of ±1 V) centering around the analog reference voltage.
Rectangular waves Asw 19 and Asw 20, undergoing an alternate change in level for every detection period T1 (for example, 10 msec) as shown in FIG. 10, are inputted from the digital processing circuit 122, as switching control signals to the analog switches SW 19, SW 20, respectively. The analog switches SW 19, SW 20 are turned on, respectively, during a time period when the rectangular waves Asw 19, and Asw 20 are being held high, respectively, while the analog switches SW 19, SW 20 are turned on, respectively, during a time period when the rectangular waves Asw 19, Asw 20 are being held low, respectively, so that the analog switches SW 19, SW 20 are alternately turned on during the detection period T1. Accordingly, a voltage corresponding to the strain along the x-axis direction, and a voltage corresponding to the strain along the y-axis, alternately appear on the common output side of the analog switches SW 19, SW 20, that is, on the input side of the ADC 125, as shown in FIG. 10. Those strain voltages are digitized by the ADC 125 to be subsequently inputted to the digital processing circuit 122.
However, in a computer provided with a conventional pressure-sensitive pointing device, a key has to be pressed additionally so as to input a command to the computer at the position of the pointer, causing a problem of low operability compared with a pointing device enabling the pointer to execute the shifting operation input (input of coordinate) and the clicking operation input.
To solve such a problem, there is proposed a pressure-sensitive pointing device provided with strain sensors for detecting loads resulting from an operation console along the direction perpendicular to an x-axis and a y-axis directions in addition to strain sensors for detecting the load along the +X direction, −X direction, +Y direction and −Y direction, thereby enabling a pointer to execute a shifting operation input and a clicking operation input (Patent Reference 2).
Patent Reference 1 (JP 7-319617A)
Patent Reference 2 (JP 2001-311671A)